Method of exfoliating and functionalizing graphite anode

ABSTRACT

A. providing an electrochemical cell with a first graphitic electrode and a second conductive electrode, wherein the first graphitic electrode is made of any one of HOPG, natural graphite, and synthetic graphite, the first graphitic electrode is held at a most positive potential, and the second conductive electrode is conductive, wherein a current passes through the electrochemical cell; B. providing an electrolyte of a solvent in the electrochemical cell, wherein the electrolyte has specific oxygen containing salts and base. Thereby, the graphitic electrode is functionalized and exfoliated by applying a voltage between the two electrodes thus producing graphene oxide.

FIELD OF THE INVENTION

The present invention relates to a method of exfoliating andfunctionalizing a graphite anode.

BACKGROUND OF THE INVENTION

Graphene materials have been at the center of large focus in the recentyears because of graphene's exceptional properties. Graphene has aunique range of properties, ranging from record electrical conductivity,thermal conductivity, mechanical stability and others. As a result, avariety of applications are envisaged in the coming years.

However, pristine graphene are difficult to process since they are notdispersed in solvents routinely used by industry making it difficult forthe widespread adoption. As such, graphene oxide is routinely used as asubstitute for this. Graphene oxide is graphene which has beenfunctionalized through epoxyl, carboxyl and hydroxyl groups. Grapheneoxide flakes are readily dispersed in variety of organic solvents andeven water due to the electrostatic interaction between theaforementioned groups and the water. Graphene oxide is also aninteresting system in its own accord. Laminates of graphene oxideprepared by means of vacuum filtration of GO dispersions are impermeableto anything other than water molecules or very few selected ions.

Graphite oxide is traditionally prepared through a chemical reactionbetween an oxidizing agent and strong acids as initially proposed by W.S. Hummers and R. E. Offeman in 1958. They used a combination ofsulfuric acid, potassium permanganate and other chemicals. Thetechnique, routinely named as Hummer's method has undergone severalmodifications since but they main theme of using strong acid andoxidizing agents has not been changed. Graphite oxide can be convertedinto single-layer graphene oxide by sonication graphite oxide usingeither a tip sonicator or placing a suspension of graphite oxide in asolvent in an ultrasonic bath.

The material prepared by the above methodology technique is unfavorablefor scale-up as there is a large amount of acidic waste andmanganese-containing waste. For reference, for 5 grams of graphene oxideproduced using this technique, several liters of water are required inorder to remove the acidic sulfuric acid from the final product thereaction and return the pH back to neutral. As the process scales intothe kilograms, several tones are required for the washing part of theexisting terms making such scale-up impractical. This is the problemthat the current invention is addressing.

The present invention has arisen to mitigate and/or obviate theafore-described disadvantages.

SUMMARY OF THE INVENTION

The primary objective of the present invention is to provide a method ofexfoliating and functionalizing a graphite anode which produces largeamounts of graphene oxide or partially oxidized graphene without theneed of either using strong acids, at the same time without requiringsonication steps. To achieve this, the exfoliation and oxidation of thegraphitic anode happens in an electrochemical cell comprising of anelectrolyte containing an oxygen containing salt and a strong base. Theuse of such salts and base in the present invention is both non-obviousand the complete opposite of the traditional Hummers methods forpreparing graphite oxide.

To obtain above objectives, a method of producing graphene oxide in anelectrochemical cell contains steps of:

A. providing an electrochemical cell with a first graphitic electrodeand a second conductive electrode, wherein the first graphitic electrodeis made of any one of HOPG, natural graphite, and synthetic graphite,the first graphitic electrode is held at a most positive potential, andthe second electrode is conductive.

B. providing an electrolyte of a solvent in the electrochemical cell,wherein the electrolyte has specific oxygen containing salt along with abase to produce graphene oxide in the electrochemical cell.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings where:

FIG. 1 is an optical image of a material on an oxidized siliconsubstrate of a method of producing a graphene oxide in anelectrochemical cell according to a preferred embodiment of the presentinvention.

FIG. 2 is a diagram showing the heigh profile of graphene oxide flakesmeasured by Atomic Force Microscopy. The height correspnds to 1 layer ofgraphene oxide.

FIG. 3 is a diagram showing the material of the method of producing thegraphene oxide in the electrochemical cell being then pushed throughappropriate membranes to prepare free-standing GO papers according tothe preferred embodiment of the present invention.

FIG. 4 is a diagram view showing Raman spectrum of a material of amethod of producing graphene oxide in an electrochemical cell accordingto another preferred embodiment of the present invention.

FIG. 5. is a diagram showing the permeation of water and isopropanolthrough free-standing graphene oxide memebranes.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described with reference to illustrativeembodiments. For this reason, numerous modifications can be made tothese embodiments and the results will still come within the scope ofthe invention. No limitations with respect to the specific embodimentsdescribed herein are intended or should be inferred.

With reference to FIGS. 1 to 5, a method of producing graphene oxide inan electrochemical cell according to a preferred embodiment of thepresent invention comprises steps of:

A. providing an electrochemical cell with a first graphitic electrodeand a second conductive electrode, wherein the first graphitic electrodeis made of any one of HOPG, natural graphite, and synthetic graphite,the first graphitic electrode is held at a most positive potential, andthe second conductive electrode is conductive

B. providing an electrolyte of a solvent in the electrochemical cell,wherein the electrolyte has specific oxygen to produce graphene oxide inthe electrochemical cell.

Regarding electrodes of the electrochemical cell, the first graphiticelectrode is a graphitic material. In one embodiment, high-qualitycrystalline may be used, in other embodiment partially exfoliatedgraphite may be used. In some embodiments, graphite already intercalatedwith salts may be used.

In some embodiments, the first graphitic electrode is contained within aplastic mesh. This facilitates the exfoliated and oxidized particles toremain in proximity of the first graphitic electrode and in closeelectrical contact with it for both further exfoliation and oxidation.

The second conductive electrode can be any material known in thoseskilled in the art as it does not play an important role in the process.Graphite, stainless steel or any conductive material polymer that iscompatible with the solvents, electrolyte may be used. In oneembodiment, both the first electrode and the second conductive electrodeare graphitic and their voltage are alternated between the firstgraphitic electrode and a second graphite electrode resulting inoxidation and exfoliation of both the first and second graphiticelectrodes. As for the electrolyte, it consists of ions in a solvent.The ions result from oxygen-containing anions. Preferable are nitrate,perchlorate, sulfrate, persulfate and phosphate anions. Nitrites,Sulfites, chlorites and phosphites may also be used in one embodiment,the electrolyte may contain a single oxygen-containing atom and inanother embodiment it may contain a combination of two or more.

The counterions (i.e., cations) play no important role in the processand can be selected from a variety of elements including, but notlimited to, lithium, sodium, potassium, ammonium, magnesium, copper,lead, cadmium, strontium, nitronium, silver caesium, barium, aluminiumand others.

The base can the hydroxides of the alkali and alkaline earth metals,which include but is not limited to sodium hydroxide, calcium hydroxide.Other examples of bases include, but not limited to, members of theArrhenius bases e.g. tetrabutylammonium hydroxide, cesium hydroxide,strontium hydroxide, barium hydroxide. The base is selected such that itis compatible with the solvent and electrode materials.

In some embodiment the concentration of the oxygen-containing salts maybe 1 mM, 0.1M, 0.2M or 0.5M, whereas the maximum concentration may be2M, or 5M. In some embodiments, the concentration of the strong base maybe 1 mM, 0.1M, 0.2M or 0.5M, whereas the maximum concentration may be2M, or 2M. In some embodiment, the concentration of the electrolyteexceed the saturation limit of the solvent.

The solvent which can be used include any organic solvent or othersolvent in which the electrolyte salts and base are highly soluble.Favorable solvent include but are not limited to: water or organicsolvents, including but not limited to acetone, isopropanol, DMSO andothers.

In addition, a working potential of the electrochemical cell will bethat requiring the oxidation and exfoliation of the first graphiticelectrode. In one embodiment, where a reference is included in theelectrochemical cell, the voltage is adjusted slightly above thispotential. In another embodiment, where the electrochemical cell onlycomprises by a single electrode an overpotential is applied which may be10V, 15V, 20V or 30V. The voltage may be kept constant or may be swappedto facilitate exfoliation at both electrodes.

The electrochemical cell is operated at a temperature which achieves thecorrect level of oxidation and exfoliation. In one embodiment, theelectrochemical cell temperature is adjusted to allow for maximumexfoliation and of the first graphitic electrode. In another embodiment,the temperature is increased to allow the more kinetic ions to causeincreased oxidation in the first graphitic electrode. Theelectrochemical cell may be operated at a temperature range of at least10 C, preferable at least 20 C. The operating temperature may be 30 C,40 C, 50 C, 60 C, 70 C, 80 C, 90 C or 100 C. Higher or lower operatingtemperatures may be used. The optimum operating temperature depends onthe combination of salt and base used in the process and on the solventused to suspend them. The higher temperatures facilitate higherconcentrations of oxygen-containing salts and base.

Preferably, the electrolyte is not consumed during the process and maybe recycled and used in further electrochemical runs. In suchembodiments the electrolyte is recovered by means of filtration, at theinterface of two immiscible liquids, by centrifugation or by techniquesknown by those skilled in the art.

Thereby, analysis of the graphene material produced by the process isroutinely conducted by means of Raman spectroscopy.

Raman spectroscopy of graphite has been performed for more than 40 yearsand it recently has been extended to single-layer graphene, itsfew-layer counterparts and to graphene oxide and few-layer grapheneoxide.

The Raman spectrum of all graphitic materials such as graphite, grapheneand carbon nanotubes is characterized by two main peaks: The D peakwhich is located at around 1350 cm⁻¹. This is a first order Raman peakwhich lies far from the Γ point of the Brillouin zone and as suchrequire defects (or sp³ material) within the basal plane of the graphitefor its activation. The G peak is located at around the 1580 cm⁻¹ and isassociated with the stretching of all sp2 rings and chains andcorresponds to a phonon at the Γ point of the Brillouin zone. The 2Dpeak is an overtone, i.e. the second order of the D peak and isactivated through a double-resonant process. In pristine graphene thewidth, and not the position, of the 2D peak can be used to unambiguouslyprove layer thickness. The position may vary due to factors such dopingor strain. For pristine graphene, the D peak is absent as there are nodefects. The G and 2D peaks are both sharp and the ratio of theirintensity (hereby denoted I(G/2D) is 1:4, that is, the intensity of the2D is nearly four times as intense as the G peak.

For graphene oxide the situation markedly changes. The introduction ofepoxyl, carboxyl and hydroxyl sp3-bonded functional groups on thesurface of graphene's basal planes means that the Raman spectrum isdifferent. The D and G peak become much broader and the intensity of the2D peaks falls well-below that of the G and D peaks. New combinationmodes are also visible which are attributed to G+D and peaks.

Although there is no strict requirement for defining graphene oxide, thematerial referred to as graphene oxide must be analogous to the materialproduced by the traditional Hummer's method and behave in a similarmanner. The most important requirement for this is thewater-dispersibility and high oxygen content due to thefunctionalization with the functional groups. This is vital since itmakes the material easy to process on an industrial level.

The produced graphene oxide material was also characterized by atomicforce microscopy in tapping mode as well as by optical microscopy todetermine contrast levels. The former technique gives height thicknessmeasurements to prove atomically thin layer thickness while the lattercan also provide quantitative information from the optical contrast theflakes provides when dispersion is drop-casted on a silicon substratewith a carefully chosen oxide layer. The term “graphene oxide” istypically referred to when the graphene oxide flake is around 1 nmthickness but we refer to “graphene oxide” as an umbrella term for bothmonolayer and few-layer (few nanometer thickness) graphene oxide flakes.The thickness of the flakes produced may vary from 1 nm to 100 nm, butpreferentially will be less than 100 nm, more preferentially less than50 nm and more preferentially under 10 nm.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

Without further elaboration, it is believed that the above descriptionhas adequately enabled the present invention. The following examplesare, therefore, to be construed as merely illustrative, and notlimitative of the remainder of the disclosure in any way whatsoever. Thepublications cited herein are hereby incorporated by reference in theirentirety.

In example 1, a nitrate salt (cation: sodium) was used in water with thebase being potassium hydroxide. The solution was stirred forapproximately 10 minutes to dissolve the salt and base in the water.Subsequently two graphite rods were inserted as electrodes. A voltage of10V was applied between the two electrodes. After about 10 seconds, thefirst graphitic electrode visibly started to exfoliate with the producthaving a characteristic brown color associated with graphene oxide. Theprocess was stopped after 30 minutes and 0.1 grams of powder wasrecovered after filtration. The powder was suspended by gently shakingin water, and 1 mL was drop-casted onto a Si:SiO₂ substrate for furthercharacterization. The optical image of the material on the oxidizedsilicon substrate is shown in FIG. 1. The Raman spectrum of the materialis shown in FIG. 2.

To determine oxygen content a graphene oxide paper was prepared bysuspending the powder in water and mildly sonicating for 10 minutes. Thematerial was then pushed through appropriate membranes to preparefree-standing GO papers as illustrated in FIG. 3. It should be notedthat the material had the characteristic mechanical strength andflexibility of GO paper. The SEM of the GO paper shows characteristicsripples and folds associated with GO paper and individual flakes are notreadily observable. The oxygen content was determined to be 30%.

In example 2, a sulfate salt was used in water with the base beingsodium hydroxide. The process run for 1 hour and 0.3 grams of materialwas recovered after filtration. The Raman spectrum of the material ispresented in FIG. 4.

It is to be noted that, the Raman spectrum is characterized by two broadpeaks centered around 1350 cm⁻¹ and 1580 cm⁻¹, the D and G peaks,respectively. The second-order peaks, the 2D are heavily quenched as thefunctionalization of graphene's basal plane results in the suppressionof the double-resonant process. This is an indication of preparation ofgraphene oxide.

The material was then mixed with water and shaken. Small amounts ofgraphene oxide dispersion was then drop-casted on a silicon wafercovered with 290 nm oxide. Tapping mode AFM reveals that the flakes havethickness of 1 nm indicating a single atomic layer of graphene oxide asshowin in FIG. 2.

In order to further testament to the quality of the graphene oxidemembranes prepared in this manner, the graphene oxide membrane wastested by x-ray diffraction. The usual graphite peak near 25o was lessprominent and sharp than graphite and a peak emerged near 11o, which isevidence of graphene oxide formation.

The membranes were also tested for permeation of water and other organicsolvents. It was found that water permeates at a fast unimpeded rate,while isopropanol, is completely blocked. This in agreement withliterature stating that such graphene oxide membranes allow permeationof water but block all other species including helium.

In example 3, a perchlorate salt was used in water (cation: lithium)with the base being tetramethylammonium hydroxide.

All of the features disclosed in this specification may be combined inany combination. Each feature disclosed in this specification may bereplaced by an alternative feature serving the same, equivalent, orsimilar purpose.

Thus, unless expressly stated otherwise, each feature disclosed is onlyan example of a generic series of equivalent or similar features.

From the above description, one skilled in the art can easily ascertainthe essential characteristics of the present invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions. Thus, other embodiments are also within the scope of thefollowing claims.

What is claimed is:
 1. A method of producing graphene oxide in anelectrochemical cell comprising steps of: A. providing anelectrochemical cell with a first graphitic electrode and a secondconductive electrode, wherein the first graphitic electrode is made ofany one of HOPG, natural graphite, synthetic graphite and partiallyexfoliated graphite the first graphitic electrode is held at a mostpositive potential, and the second electrode is conductive; B. providingan electrolyte in the electrochemical cell comprising a solvent, whereinthe electrolyte contains specific oxygen containing salts and a strongbase facilitating exfoliation and oxidation of the graphite electrode..2. The method of producing the graphene oxide in the electrochemicalcell as claimed in claim 1, wherein the graphitic electrode ispretreated by any one of sonication, ultrasound, and chemical.
 3. Themethod of producing the graphene oxide in the electrochemical cell asclaimed in claim 1, wherein the second conductive electrode is aconductive polymer or a metal.
 4. The method of producing the grapheneoxide in the electrochemical cell as claimed in claim 1, wherein thesolvent is water.
 5. The method of producing the graphene oxide in theelectrochemical cell as claimed in claim 1, wherein the solvent is anorganic solvent.
 6. The method of producing the graphene oxide in theelectrochemical cell as claimed in claim 1, wherein the solvent is amixture between water and an organic solvent.
 7. The method of producingthe graphene oxide in the electrochemical cell as claimed in claim 5,wherein the solvent is any one of acetone, isopropanol, ethanol andmethanol.
 8. The method of producing the graphene oxide in theelectrochemical cell as claimed in claim 1, wherein the electrolyteconsists of salts where in an anionic part is any one of sulfate,nitrate and chlorate.
 9. The method of producing the graphene oxide inthe electrochemical cell as claimed in claim 1, wherein the electrolyteconsists any one of persulate, perchlorate, and perphosphate.
 10. Themethod of producing the graphene oxide in the electrochemical cell asclaimed in claim 9, wherein the cationic part of the salt is selectedfrom sodium, potassium, and ammonium.
 11. The method of producing thegraphene oxide in the electrochemical cell as claimed in claim 10,wherein the cationic part of the salt is selected from sodium,potassium, and ammonium.
 12. The method of producing the graphene oxidein the electrochemical cell as claimed in claim 1, wherein theelectrolyte further consists of an Arrhenius base.
 13. The method ofproducing the graphene oxide in the electrochemical cell as claimed inclaim 12, wherein the base is any one of potassium hydroxide, sodiumhydroxide, and tetraethylammonium hydroxide.
 14. The method of producingthe graphene oxide in the electrochemical cell as claimed in claim 1,wherein the electrolyte is recovered by i) filtration, ii) interface oftwo immiscible liquid, and iii) centrifugation.
 15. The method ofproducing the graphene oxide in the electrochemical cell as claimed inclaim 1, wherein the graphene oxide is used to prepare water-dispersiblesuspensions.
 16. The method of producing the graphene oxide in theelectrochemical cell as claimed in claim 1, wherein the graphene oxideis used to prepare freestanding graphene oxide membranes.
 17. The methodof producing the graphene oxide in the electrochemical cell as claimedin claim 1, wherein the graphene oxide is used to prepare compositematerials.